The present invention relates to a communication apparatus and communication system, such as a communication system including a base station and a mobile station according to a CDMA (Code Division Multiplex Access) communication scheme.
In digital mobile wireless communication approaches, fading causes SNR (Signal-to-Noise Ratio) to worsen, leading to deterioration of communication quality. Spread spectrum systems like a CDMA communication system also have frequency selective fading due to multi-paths and fading caused by the high-speed movement of a mobile station to a base station, which are troublesome.
To solve the problems and enhance the communication quality, there has been known a method of stabilizing the receipt level at a base station to a constant by controlling transmission power (e.g., reverse linkage power control) at a mobile station, during wireless transmission from the mobile station to a base station as described in the following reference.
Andrew J. Viterbi, `CDMA Principle of Spread Spectrum Communication`, pp. 113-119, published by Addison Wesley, 1995
A wireless communication system using the reverse link power control method is shown in FIG. 2. Base station 1 receives a signal influenced by fading. The signal is demodulated into a receipt data signal by digital demodulator 10, and the power (receipt power level) of the decoded receipt data signal is measured by power measurement circuit (POW) 11. The measured power is provided to power control information generation circuit 12. In power control information generation circuit 12, the difference between the receipt power level and a targets power level held in target level holding unit (M) 14 is calculated by subtractor 13. According to the signal of the difference, one bit of transmission power control information is formed, which indicates whether the transmission power at mobile station 2 should increase or decrease. The transmission power control information is multiplexed with transmission data by digital modulator 16, thereby being digitally modulated to be sent to mobile station 2.
In mobile station 2, a receipt signal arrived via a transmission path from the base station is demodulated. Transmission power control information is separated from the obtained receipt data signal, and it is provided to inverse quantization circuit (Q.sup.-1) 22. Inverse quantization circuit 22 outputs, for example, a power modification signal of +0.5 dB to increase the power, or -0.5 dB to decrease. The outputted signal is added to the current power control value outputted from one sample delay circuit (Z.sup.-1) 24, whereby a new power control value can be generated. Linear region conversion circuit (EXP) 25 converts it into a power control value in the linear region, which is then applied to a control terminal of transmission power amplifier (A) 26, which is a variable amplification unit. Thereby, the power of a transmission signal is directed toward base station 1.
In summary, in the above processes, the difference between a receipt power level and the target power level is quantized with one bit and then transmitted, and the transmitted signal experiences integral calculus to restore the fading signal, which is canceled by controlling variable amplifier 26 through the inverse characteristics of the processes. The processes assumes that a fading signal does not change for a short period of time. Herein, the fading signal denotes the characteristic of fading. The fading signal is defined as a ratio of a receipt power Pb of the base station to a transmission power Pm of the mobile station; that is to say, the fading signal X=(receipt power Pb)/(transmission power Pm). The fading signal changes with time. Therefore, the function of the fading signal is defined as X(n), where n indicates a time.
However, on the assumption, transmission power control cannot follow rapid change in fading which may occur during high speed movement of the mobile station.
Moreover, though transmission power control in the mobile station needs transmission power control information from the base station, there lies a process delay period of time by the time that transmission power control information from the base station reaches the mobile station. Since a large fading may occur for a large process delay period of time, the conventional method of controlling transmission power cannot perfectly follow such a fading, whereby a large error of transmission power control deteriorates communication quality.